Compensating device and method of compensating



Oct. 27, 1931. w. LIKRAHL 1,829,225

COMPENSATING DEVICE AND METHOD OF COMPENSATING Filed Dec. 30, 1927 r-1r-\fi 25 T29 IV '26 1 1 I 1% voLTs 4.2, VOLTS 45VOLT$ g voL-rs r ZO /9 INVENTOR MILTER lows KIWI/IL A RNE Patented Oct. 27, 1931 UNITED STATES PATENT OFFICE WALTER LOUIS KRAHL, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO AROTURUS RADIO TUBE COMPANY, OF NEWARK, NEW JERSEY, A CORPORATION OF DELAWARE OOMPENSATING DEVICE AND METHOD OF COMPENSATING Application filed December 30, 1927. Serial No. 243,501.

This invention relates to alternating current tubes of the heater type and particularly to a means and method for compensating for the alternating current potential drop between the grid and the cathode of the tubes.

In some tubes of the heater type it has been the custom to connect the cathode directly to the heater circuit within the tube so that one of the terminals of the tube is used for a connection to the cathode and also for one side of the heater circuit. The grid return is necessarily connected to the cathode lead, and inasmuch as a portion of this lead carries alternating current for the heater circuit, the resistance of this portion of the lead causes an alternating current potential drop between the grid and cathode of the tube Which produces a hum in circuits designed for very efficient amplification.

It is one of the objects of the invention, therefore, to provide a. means and a method for eliminating this drop in potential in the common cathode and heater current lead of an alternating current heater type tube to eliminate the hum caused thereby.

Another object of the invention is to provide a means which will simultaneously eliminate the alternating current potential drop in the common cathode and heater current lead of a heater type alternating current tube and produce a negative bias for the grid of the tube.

Another object of the invention is to provide an easily constructed adjustable device for eliminating the alternating current potential drop between the grid and cathode in an alternating current heater type tube having a common connection between the V cathode and heater circuit which may be easily installed in the circuit and which may be readily adjusted.

Another object of the invention is to provide a means and a method for connecting the grid return of a heater type alternating current vacuum tube having a common connection between the cathode and heater circuit to a point connected to the cathode and having exactly cathode potential.

Other objects of the invention will be apparent as the description thereof proceeds.

One embodiment of the invention has been illustrated in the accompanying drawings in which:

Fig. l is a circuit diagram of a three tube radio set incorporating the invention;

Fig. 2 is a diagram of a single tube with certain parts of the circuit omitted in order to illustrate the principle involved in the invention;

Fig. 3 is a modification of the construction shown in Fig. 2; and

Figs. 4: and 5 are further modifications of the invention.

The invention is shown in connection with a radio circuit having aradio frequency amplifier tube I, a detector tube II and an audio frequency amplifier tube III. The usual radio frequency transformers and 11 are connected respectively before tubes I and II, while the audio frequency transformer 12 is connected before the audio amplifier tube III in a manner well known in the art. Tuning condensers 13 and let are connected respectively across the secondaries of the transformers 10 and 11 to provide a means for tuning the set in the usual manner.

' The alternating current tubes illustrated form the subject matter of my co-pending application Vacuum tube construction and circuit therefor Serial No. 156,947, filed December 24, 1926, and the operation thereof need not be explained here. This tube comprises a grid 15, a plate 16, a cathode 17 and a heater filament 18 which is connected to the cathode 17 as indicated diagrammatically in the drawings. The tubes are all connected to two common leads 19 and 20 which are connected respectively to the filament terminals 21 and 22 of the tubes. The leads 19 and 20 are connected to the secondary of a trans former 23, the primary of which is connected to a source of alternating current supply.

The grid returns for the tubes I, II and III are brought to a common lead 24 which is connected to the negative terminal 25 of a C battery having approximately one and one-half volts, the positive terminal of which is connected to the B terminal 26 of the set. The grid of the detector tube, however, is maintained at a positive potential by connecting the grid in series with a grid leak 27 to four and one-half volts positive at 28 by the lead 29. A connection 30 isnow necessary between the B terminal 26 and the heater the heater circuit, but certain difiiculties arise I in connection with..the circuit as described above. Any difference in alternating current potential between the cathode and grid of a tube will set up a hum in the plate circuit of the tube, which when amplified sufficiently may become distinctly audible so as to create considerable annoyance to anyone listening to the signals reproduced by the set. Such a hum is not produced by any tubes prior to the detectortube where the hum usually originates; It .is then amplified by the audio frequency transformer and tube, and especially where a number of such amplifying tubes are used it is increased until it becomes objectionable. This difference in potential be tween the grid and cathode of the tubes is. brought about by the fact that the alternatingcurrent traverses a portion ofthe cathode circuit, which portion corresponds to that part of the cathode lead which lies between the connection of the heater filament with the cathode inside the tube and the connectionhof the grid return lead 30 to the lead 20, or in other words, that part of the cathode lead which is common to both circuits.

In order to compensate for this alternating current potential drop I have found that a high resistance 31 may be placed across, the

alternating current leads 19 and 20 and. the

hlgh In Order to use as httle as posslblgofi 33 included in the grid circuit to providea path for the incoming signal current, although such a' condenser is not considered grid return lead 30 connected to a point on the resistance between which and the end of the resistance nearest the cathode of the tube the alternating current potential drop will equal the alternating current potential drop in the'lead 20 fromthe end of there, sistance to the connection of the cathode with the heater filament. It is, of course, preferable to provide such a resistance relatively the alternating current, and therefore I have found it desirable to use two resistances, one

32 of approximately one thousand ohms and at the end of this resistance nearest the lead 20a relatively small resistance 33 of approxi: mately one ohm, to which a movable arm 34 may be attached, which forms the connection for'the grid return circuit 30. l/Vhile it is not necessary to use the exact valuesjust referred to, the resistance in the common cathode and heater current lead is so slight (merely the resistance of the wire) thattheproportion of the small resistance 33 to the whole resistance 31 should be approximately as one is to one thousand. 7

Inasmuch as the cathode of the tube is connected to the B terminal through a portion of the resistance 33, the cathode will be more vpositive than if this resistance were not used,

andthis has the efifect of increasing the negative bias of the grid. Consequently, if the resistance 33 is made great enough, sufli- .cient negative bias will be placed on the grid of the tube so that a separate battery will not have to be used. To accomplish this the total resistance 31 wo'uldhave to be considerably larger than the figures given, as the resistance be adjusted once forthe particular tubes used in the set; i u I v Another .method of connecting the resist ance is illustrated in Fig. 3 where a rheostat .35'is shownusedinplace of a potentiometer.

In this case the fixed resistance 32 is connected tothe arm 36 of a rheostat together with the grid return lead 30 as indicated and thecoil 37 of the rheostat, corresponding to the resistance 33, is connected to the other side of the filament. current supply circuit 20.

A still further method of introducing the resistance into the circuit is illustrated in Fig. 5 in which the grid return'lead 30 is connected to the arm 41 ofarheostat42 having a coil '43 of high resistance corresponding to the resistance 32 which is connected to the lead 19. A small fixed resistance 44 corresponding to the resistance 33 is connected between the'arm 41 and the lead 20. When the arm-41 is moved the relation between the two resistances 43 and 44'varies which varies the potential drop across the resistance 44 and a point may therefore, be found at which the alternating current potential. drop ,in the cominon lead is balanced out. a

A condenser 40 (Fig. 2) maybe connected if desired, across theportion of the resistance to be necessary when the included resistance issmall. I,

It is possible to completely do away with the common heater and cathodelead andso eliminate the cause of the hum by providing an entirely separate cathode, connection 38 (Fig. 4) for the tube without changing any of the elements and it may be desirable to'do this althoughlan additionalterminal 39 on r the tube would haveto be provided. 'It'is wlthin thes'piritlof the invention however to cover such a construction. Also, the invention contemplates the connection of the grid return circuit directly to a point on the secondary of the alternating current transformer having exactly cathode potential, although such a point would be extremely ditficult to find.

It is to be understood that the present invention is only necessary where a very efficient circuit is employed, as in the ordinary circuit low frequencies are not amplified suificiently to make the hub objectionable. Although the hum originates in the cletector tube and one potentiometer is usually sufiicient to eliminate it, it may be that in certain sets enough hum is produced in the audio amplifier tubes to be objectionable, and in this case it may be desirable to use a potentiometer across the heater circuit of each of the detector and audio frequency tubes.

While I have illustrated and described certain embodiments of the invention, I do not desire to limit myself to the specific construction shown and described, but to interpret the invention broadly within the scope of the appended claims.

Having thus described my invention, what I claim is:

1. In a radio circuit comprising alternating current heater type tubes in which the cathodes and heater circuits have a common connection, said connection introducing an alternating current potential drop therein, and means for compensating for said alternating current drop of said common connection.

2. In a radio circuit comprising alternating current heater type tubes in which the cathodes and heater circuits have a common connection, said connection introducing an alternating current potential drop therein, and means for compensating for said alternating current drop of said common connection, said means comprising a resistance in one of the circuits substantially equal to the resistance in said common connection.

3. Means for compensating for the voltage drop on the common cathode and heater current lead in a radio set using heater type thermionic tubes having a common connection between the cathode and heater comprising a relatively high resistance across the heater circuit, said resistance having a connection for the grid return of the set at a point between which and the said common lead the voltage drop is equal to the voltage drop on the said common lead.

4. Means for compensating for the voltage drop on the common cathode and heater current lead in a radio set using heater type tubes having a connection between the cathode and heater comprising a relatively large resistance connected across the heater circuit including a small resistance having a ratio of approximately one to one thousand to said large resistance and a variable arm contacting with said small resistance and adapted to be connected to the grid return of the set.

5. In a radio circuit using alternating current tubes of the heater type having a connection between the cathode and heater, means to compensate for the voltage drop in the common cathode and heater current lead of the detector tube comprising a potentiometer having a variable arm adapted to sweep over approximately one-one thousandth of its total resistance connected across the heater circuit leads to said tube and having the grid return of said tube connected to said arm. i

6. In a radio circuit using alternating current tubes ofthe heater type having a connection between the heater and cathode, means to compensate for the drop in potential in the common cathode and heater current lead ct any one tube comprising a relatively high resistance across the heater leads of that tube and a connection for the grid return of that tube at a point between which and the said common lead the resistance is sufficient to compensate for the alternating current potential drop in the common current and cathode lead.

7. In a radio set comprising alternating current tubes of the heater type having a common cathode and heater connection. means to connect the grid return lead to the cathode at a point having exactly cathode potential and to simultaneously place a nega' tive bias on the grid of the tube.

8. Means to simultaneously compensate for the voltage drop on the common cathode and heater current lead in a radio set using heater type alternating current tubes having a connection between the heater and cathode and create a negative grid bias which comprises a resistance across the heater current circuit having the grid return connected to it at a point having cathode potential.

9. The method of compensating for the voltage drop in the common heater current and cathode lead in a radio set employing alternating current heater type tubes having a connection between the cathode and heater which comprises introducing an opposing alternating current into the grid return lead of sufiicient strength to balance the voltage drop in the said common lead caused by the resistance thereof.

10. In a radio circuit comprising alternating current heater type tubes in which the cathodes and heater circuits have a common connect-ion, said connection introducing an alternating current potential drop therein, and means for compensating for said alternating current drop of said common con nection, said means comprising a resistance in the cathode circuit substantially equal to the resistance in said common connection.

11; The method of simultaneouslyvcreating a grid bias on an alternating currenttube' of the heater type having a connection. be tween the cathode and heater and compensating for the alternating current Voltage drop on the common cathode and heater current lead of the tube which comprises connecting. a potentiometer across the heater leads of the tube, connecting the grid return of the tube to the arm of the potentiometer and ing cathode potential. a .12. In a radio circuit comprising alternating current heater type tubes in which the cathodes and heater circuits have a common connection, said connection introducing an alternating current potential drop therein, and means for compensating for said alternating current drop of said common connection, said means comprising a difierential resistance across the filament I circuit with the low resistance in series with the cathode circuit. 1

13. In aradio circuit comprising alternating current heater type tubes in which the cathodes and heater. circuits have a common connection, said connection introducing an adjusting the potentiometer to a point havalternating current potential drop therein,

' in said common connection.

and means for compensating for said alternating current drop of said "common con-. nection, said means comprising a differential resistance across the filament circuit with the low resistance in series with the cathode cirouit andsubstantially equal to the resistance 14. An alternating current radio set utilizing heater ty'pe tubes, comprising a pair of wires for conducting current to the heater terminals of said tube, a relatively large j resistance connected to one of said wires, a

relatiyely small resistance connected between the end of said first resistance and the other of-said pair of wires, a third wire connected to a point on said small resistance, and means 1 to vary the 'reslsta'nce to wh1ch said wire is attached.

In testimony whereof, I afiix my signature. V

- WALTER LOUIS KRAHL. 

